What determines if you can flee from a wild encounter?

Question

My Crobat can always, always run away from wild Golbat. But occasionally, my Magneton will not be able to run away on the first attempt and succeed on the second. Rarely, my Gastrodon will not be able to escape for two or three turns.

What determines a Pokémon's ability to run away from a wild encounter?

P.S. This has nothing to do with Mean Look, so don't use that as your answer.

Comments

http://bulbapedia.bulbagarden.net/wiki/Escape this tells you but there is no info for new generations

Answer 1

Escape is not guaranteed however; whether the player is successful at running away from the battle is determined by a calculation involving the speed of the combatants.

It involves speed and an escape formula.

Escape formulas:
For Gen 1 and 2:

>Under normal circumstances, the chance of escaping is determined by the formula Escape
, where
A is the current Speed of the player's active Pokémon,
B is the opposing Pokémon's Speed divided by 4, mod 256, and
C is the number of times the player has tried to escape during the battle (counting the current attempt). If player's Pokémon attacks, this number is set to 0.
If F is greater than 255, the player escapes automatically. Otherwise, a random number is generated between 0 and 255. If that number is less than F, the player escapes. If not, the escape fails and the player's Pokémon does not make a move that turn. If B is equal to 0, escape is automatically a success.

For Gen 3 and 4

>Under normal circumstances, the chance of escaping is determined by the formula , where
A is the unmodified Speed of the player's active Pokémon,
B is the opposing Pokémon's umodified Speed
C is the number of times the player has tried to escape during the battle (counting the current attempt).
The result is mod 256.
A random number is generated between 0 and 255. If that number is less than F, the player escapes. If not, the escape fails and the player's Pokémon does not make a move that turn. If B is 0, it's set to 1.

Basically, it has to do with speed.
Hop I helped!
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